The present invention relates to a jet pump including a nozzle and an ejector tube for use in back pulsing a cylindrical pleated paper filter element in a filter assembly to blow the dust off the cylindrical wall of the element and thereby provide continuous cleaning of the filter element. The filter assembly for which this ejector tube was particularly designed is similar in its general operation to that described in British Pat. No. 1,516,721 which, in turn, is quite similar in operation to the bag house filter assemblies which have been known for many years.
The more recent filter assemblies of this type comprise a large housing which is partitioned into a lower dirty air portion and an upper clean air portion. A plurality of rows of cylindrical pleated filter elements are removably mounted in the lower portion of the housing such that the open upper end of the filter elements are in sealing engagement with the partition between the upper and lower portions of the housing. Apertures are provided in the partition in axial alignment with the open end of the secured filter element communicating the interiors of the filter elements with the upper clean air portion of the housing. Dust laden air is directed into the lower portion of the housing where it passes through the walls of the filter element, depositing its dust, and upwardly through the interior of the filter element into the clean air portion of the housing. The air is then exhausted through an air outlet whereupon the exiting clean air can be recirculated for ventilation, directed to the air intake of a large gas turbine or used for any number of purposes.
To prevent the individual filter elements in such an assembly from becoming overladen with dust particles, each filter element is repeatedly cleaned by back pulsing a jet of air downwardly through the interior of the filter element opposite to the direction of general air flow. This back pulsing forces the dust particles which have accumulated on the filter elements outwardly therefrom whereupon they fall to the bottom of the lower portion of the filter housing where they can be accumulated in a hopper for disposition.
In back pulsing the paper pleated filter elements of the type used in such filter assemblies, which are more fully described later herein, it is necessary to pump a relatively large volume of air in a quick pulse through the filter element to remove the dust which has collected between the narrowly spaced pleats of the filter media. While the continuous air flow through the filter assembly may be on the nature of 400-500 cubic feet per minute, to be effective, this back pulse which is only about 0.5 seconds in duration, should be at least 1250 cubic feet per minute against a head of approximately 10 inches of water. Accordingly, it has been necessary to develop a suitable jet pump capable of providing such a high volume and velocity back pulse to adequately clean these pleated paper filter elements.
The jet pump which has heretofore been used on such filter assemblies are essentially the same as those used in the conventional reverse pulse "bag house" filter assemblies. These pumps comprise a venturi tube mounted above the cylindrical filter element. To clean the filter bag in the bag filter assemblies, a short burst of compressed air was directed into the outlet end of the venturi tube. This momentary jet of high velocity high entrained and forced secondary air into the bag which caused it to inflate and then snap onto its wire cage support. This change in shape and snap action of the bag dislodged the accumulated dust and filter. However, when used with the more rigid pleated paper filter element which do not undergo this change of shape during cleaning, it has been found that the venturi tubes are too inefficient to properly blow the collected dust from the narrowly spaced pleats in the filter element. Accordingly, in order to obtain the full benefit of the improved efficiency of the cylindrical pleated paper filter elements in a back pulsing filter assembly, it is necessary to develop a more efficient jet pump. The ejector tube described herein provides such a pump.